I can hardly bend over now. Are there any stretches I can do to alleviate the stiffness in my lower back? I feel like I can't relax it.
The deadlift is a high muscle tension grind exercise, where you move a weight from point A to point B. The lower back can very easily be fatigued in this type of movement and be the weak link for many people as it's under constant tension. Stiffness is also a normal symptom of doing high tension type training as your muscles can lack the ability to fully relax.
You need to include some dynamic exercises which mimic a movement similar to the deadlift, like the barbell high pulls from the hang or floor position using a light weight. In this exercise, you generate a lot of force and power to lift the barbell dynamically and then almost relax to drop the barbell under control. This type of dynamic movement is really great in relieving any stiffness or tension in muscles as you have to forcefully relax and contract your muscles to do this exercise correctly.
Strength without endurance, is as useless as, endurance without strength
Try Keystone Deadlifts. If done correctly you should feel no discomfort in your lower back and get a nice burn in your hamstrings and glutes. Allows more weight to be used without any lower back trauma.